The small loss in bone density that is a risk of Truvada (tenofovir/emtricitabine) as pre-exposure prophylaxis (PrEP) reversed among a group of young adults after they stopped taking the drug. Researchers in the iPrEX Study Team examined the changes in bone density in members of a substudy of the global, blinded iPrEx trial that first proved PrEP efficacy among men who have sex with men (MSM) and transgender women in 2010, and which was followed by an open-label extension trial (iPrEx OLE).

Robert M. Grant, MD, MPH, a professor at the University of California, San Francisco, and the head of the iPrEx trial, presented findings from the study at the 2016 Conference on Retroviruses and Opportunistic Infections (CROI) in Boston.

There was a gap between the iPrEX and the iPrEx OLE trial. This provided the researchers an opportunity to study how bone density shifted after the participants stopped taking the study drug.

The bone density substudy included 498 participants, whose bone mineral density (BMD) was tested every 24 weeks during the iPrEx trial, 24 weeks after they were no longer given Truvada, and at the beginning of the subsequent open-label extension trial. Among those who had BMD tests at the start of iPrEx, the median age was 29.

A total of 352 (71 percent) of the substudy participants had BMD tests conducted 24 weeks after they no longer received Truvada, and 289 (58 percent) had the tests at the beginning of iPrEX OLE, which began a median 73 weeks after the end of the end of participants’ time in iPrEx.

The bone density loss the participants experienced during iPrEx stopped after they were no longer provided Truvada. The average BMD loss recovered completely in the spine six months after the end of iPrEx, and recovered completely in both the hip and spine by the beginning of iPrEx OLE. This evidence of BMD recovery persisted after the researchers adjusted the data for various differences among the participants, including how frequently they took the daily Truvada regimen.

Bone mineral density also recovered among participants younger than 25, whose BMD contintued to increase even beyond the point seen at the outset of their participation in iPrEx. BMD tends to increase in young people until about the age of 25, where it achieves a certain set-point for adult life. Previous research has raised concerns that PrEP may stunt bone growth among young people and result in a lower ultimate set-point. This study, which included particpants 18 years of age and older, somewhat assuages such concerns, but does not address the effects of PrEP use among young adults who take Truvada for much longer periods than those seen in iPrEx, nor does it address concerns about PrEP use among people under 18. (Participation in iPrEx lasted for a median 1.2 years and a maximum 2.8 years.) According to Grant, other research continues to investigate the quesiton of PrEP’s longterm effects on young people’s BMD.